Preformed magnetic core structure



Aug. 30, 1966 A. F. MITTERMAIER PREFORMED MAGNETIC CORE STRUCTURE 4 Sheets-Sheet 1 Filed May 26, 1965 FIEZ IR S/ ADHES/ V5 Aug. 30, 1966 A. F. MITTERMAIER FBEFORMED MAGNETIC CORE STRUCTURE 4 SheetsSheet 2 Filed May 26, l965 INVENTOR. ArmzzF/WZterWakn BY 2 flfzar'ney,

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United States Patent 3,270,308 PREFORMED MAGNETIC CORE STRUCTURE Armin F. Mittermaier, Fort Wayne, Ind., assignor to General Electric Company, a corporation of New York Filed May 26, 1965, Ser. No. 461,602 8 Claims. (Cl. 336-217) This application is a continuation-in-part of my copending application Serial No. 398,708, now abandoned, filed September 23, 1964, and assigned to the assignee of the present application. The invention relates generally to cores for electromagnetic inductive apparatus. More particularly, it relates to a type of core comprised of individually preformed laminations stacked so that the lines of magnetic flux pass through the lamination strip material in the most favorable magnetic direction or, in other words, in the direction of the orientation of the crystal grains of the material.

In a commonly used laminated magnetic core, the core is formed by stacking lamination punchings of various shapes, such as I-shaped and E-shaped punchings. For example, the E-shaped lamination punchings are stacked one on top of each other to a predetermined height and butted against or interleaved with a stack of I-shaped punchings of the same height to form a closed magnetic circuit. Magnetic cores employing such flat lamination punchings are economically attractive since they utilize relatively simple punchings having two identical shapes which can be readily fabricated and held to close tolerances to provide good butt or interleaved joints between the lamination stacks. However, the fiat lamination punchings cannot be arranged in a closed loop type of core construction so that the lines of magnetic flux will always pass through the steel in the direction of the preferred orientation of the grains or in the direction of cold rolling. An inherent disadvantage of such magnetic cores is that the core losses are relatively high, and the user of the device must pay for the increased power consumption resulting from these losses. Also, flat lamination punchings require the use of relatively expensive dies and cannot readily be produced without scrap.

There have been many core constructions proposed in the past to better utilize the magnetic properties of strip material, such as cold rolled silicon strip steel. In some core constructions the laminations are superposed on each other and nested one lamination layer within the other with the individual laminations being bent around or wound around the core corners. In such core arrangements the magnetic flux will travel through the core in a lengthwise direction or in the direction of rolling. It is necessary in such bent or wound core constructions that the silicon strip material should not be bent beyond its elastic limit. As a practical matter, a considerable amount of deformation is required to complete the assembly of core sections of such conventional bent or wound core constructions, and the presence of this deformation in the core causes the core losses to increase.

Another problem which has been encountered in practice with magnetic cores employing a bent or wound core construction is that it has been difficult to obtain proper joints between adjoining ends of the core sections so that core losses can be effectively minimized.

3,270,308 Patented August 30, 1986 It will be appreciated that the reluctance across a separated butt joint between laminations is many times the reluctance of a joint in which a good steel-to-steel contact is achieved. In order to reduce the reluctance in the core resulting from such separations, staggered and diagonal butt joints have been proposed. Such joints have not been readily achieved in practice without causing the manufacturing costs to exceed reasonable or commercially acceptable limits.

Accordingly, a general object of my invention is to provide an improved core construction that can be fabricated in many sizes and shapes without need for dies and without scrap.

It is another object of my invention to provide a new and improved magnetic core construction utilizing laminates preformed from strip material wherein the lami nates are readily adaptable to semi-automated and completely automated manufacturing techniques.

A further object of my invention is to provide an improved magnetic core construction of preformed laminates wherein the manufacture of the individual laminates is greatly facilitated without excessively deforming the strip material.

It is also another object of the invention to provide a magnetic core construction characterized by an improved joint arrangement.

A still further object of my invention is to provide an improved magnetic core construction wherein the core losses are effectively minimized.

In accordance with one form of the invention I have provided an improved core structure for electrical inductive apparatus which is comprised of a plurality of successively superposed laminate layer groups. Each laminate layer group includes at least one essentially rectangular laminate layer formed of a pair of preformed laminates. Each laminate has a pair of leg portions disposed essentially in normal relation with respect to each other. Further, each laminate is formed from a thin strip of magnetic material having the most favorable magnetic direction lengthwise of the strip. Each leg portion of a laminate in a successive layer of one laminate group has a length that differs by one thickness of the strip material from the length of the corresponding leg portion of the laminate in the preceding layer of the laminate group. Each leg portion of the laminates that form the first layer of one successive laminate group has a length that differs from the length of the corresponding leg portion of the laminates in the adjacent laminate layer of the preceding laminate group by an amount essentially equal to the number of layers in the successive laminate group multiplied by the thickness of the strip material plus one additional thickness.

In a more specific aspect of the invention the core structure is comprised of successively superposed rectangular laminate groups. Preferably, each of the rectangular laminate groups is formed of n laminate layers, where n is an integer greater than one. Also, each of the n laminate layers includes a pair of essentially L-shaped laminates having a first and a second leg portion disposed in a substantially normal relation with respect to each other. The first and second leg portions of the laminates in a successive laminate layer of one laminate group have lengths respectively that differ from the lengths of the corresponding first and second leg portions of the laminates in a preceding layer by one strip material thickness. Further, the first and second leg portions of the laminates in the first layer of one successive rectangular laminate group have lengths that differs respectively from the lengths of the first and second leg portions of the laminates in the adjacent layer of the preceding rectangular laminate group by an amount equal to essentially (n+ 1) thicknesses.

The improved laminated core construction makes it possible to fabricate preformed laminates for magnetic cores on a mass production basis and provide a magnetic core assembly characterized by appreciably reduced core losses. Further, with the improved core construction a stepped joint configuration is rendered practicable in a magnetic core utilizing individually preformed laminates.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention itself, however, both as to organization and method of operation together with further objects and advantages thereof may be best understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of a magnetic core embodying one form of the invention and showing a coil assembly mounted thereon;

FIGURE 2 is an enlarged sectional view taken generally along lines 2-2 of the magnetic core shown in FIGURE 1;

FIGURE 3 is a fragmentary view of the upper right hand corner of the magnetic core shown in FIGURE 2, the view being enlarged to illustrate the details of the joint assembly;

FIGURE 4 is an exploded view illustrating the manner in which two laminate layer groups of a core section shown in FIGURES l,3 are arranged;

FIGURE 5 is an illustration of a core member pulled apart to illustrate the stepped joint arrangement of the improved core construction and the manner in which the coil assembly is placed on the winding leg;

FIGURE 6 is a front elevational view of a magnetic core and coil assembly embodying the invention, the coil assemblies being shown in section; and

FIGURE 7 shows three curves representing a plot of the flux density in kilolines per square inch versus core loss in watts per pound of core material, curve A representing core loss characteristics for a conventional 5 l .v.a. transformer with a magnetic core formed of E-I laminations, curve B representing the core loss characteristic for a transformer with a 5 k.v.a. rating and having improved magnetic core formed with preformed L-shaped laminates and employing one laminate layer per step in the joint configuration, and curve C represents a core loss characteristic of a five k.v.a. transformer utilizing an improved magnetic core formed of preformed L-shaped laminates and utilizing four laminate layers per step.

Referring now more in detail to FIGURES l-S of the drawings, I will now more fully describe a laminated magnetic core embodying one form of the invention. The magnetic core 10 is formed of a plurality of preformed L-shaped laminates made of suitable magnetic material, such as cold rolled silicon steel. The relative arrangement of the individual laminates 11, 12 can be best seen in FIGURES 3 and 4.

In FIGURES 1 and 2 the coil assembly 13 is shown mounted on the center winding 14. Suitable leads 15, 16, 17 and 18 extending from the coil assembly 13 are provided for connecting the electrical windings of the coil assembly 10 in an external electrical circuit. Any suitable number of layers may be used depending on the size of the magnetic core 10 which is desired in a particular application.

As is best seen in FIGURES l and 2 the magnetic core 10, as exemplified in this specific embodiment of the invention, is comprised of two identical core mem bers 19 and 20 which are joined at the sides to complete 4 the core structure and provide the center winding leg 14. In FIGURE 5 the core member 20 is shown separated at the joints formed between the two segments of the core member 20. The core members 19 and 20 are separated at the joints to allow the coil assembly 13 to be placed on the center winding leg 14.

Having more specific reference now to FIGURES 3 and 4, will now more fully describe the joint arrangement of the magnetic core embodying one form of the present invention and utilizing a four layer per step joint pattern. The magnetic core 10 is formed of rectangular laminate layer groups 25, 26, 27, 28, 29 and 30, only two of which groups 25 and 26, are shown in the exploded view of FIGURE 4. As is best seen in FIGURE 4, the laminate groups 25 and 26 are comprised of the L-s-haped sections 31, 32 .and 33, 34 respectively. It will be seen that the laminate section 31 consists of a stack of L-shaped laminates 11, 11a, 11b and 11c. Similarly, the other L-shaped section 32, which is joined with section 31 to form the laminate group 25, is formed by the laminates 12, 12a, 12b and 120. In particular, it will be noted that a laminate layer 35 in laminate group 25 is formed by joining the laminates 1.1 and 12 so that one end of each of the laminates 11, 12 provides an overlap portion 8 and 9, respectively, equal to the sum of the thicknesses of the successive laminates in the laminate group 25 which abut the overlap portions 8 and 9 of the laminates 11 and 12. Further, it will be noted that laminate 12 is formed with leg portions 41 and 42 which are essentially at right angle or in normal relation with respect to each other. In some applications it may be desirable to provide an angle between the leg portions 41 and 42 that is slightly less than ninety degrees so that a gripping action between successive laminates may be obtained.

In FIGURE 3 I have shown an enlarged corner fragment of the magnetic core member 20 to show the complete stepped joint con guraition. It will be noted that four laminate layers are used in each layer group. Although in the specific embodiment of the invention shown each layer group consisted of four layers comprised of eight laminates, it will be understood, however, that a different number of layers per group can be readily utilized in the constructions embodying the present invention.

In the embodiment of the invention illustrated in FIG- URES l-6 an epoxy resin adhesive 40 was applied at the stepped joint to hold the layer groups and the individual laminates in assembled relation. It will be appreciated, however, that other means such as a mechanical frame may be utilized to hold the magnetic core in assembled relation.

To produce the individual laminates for the core mem bers 19 and 20, the laminates were formed by progressively changing the lengths of the L-shaped laminates used in the successive layers by increments essentially equal to the one or more thicknesses of the strip material used in the fabrication of the'laminates, as now will be more fully described. In the four layer step joint configuration used in the specific embodiment of the inventron shown in FIGURES 1-4, the first L-shaped laminate 1 1 of core member 20 was formed by indexing a strip of magnetic material to a predetermined length, bending at the end of the first indexed length, feeding out a second indexed length and then severing the preformed laminate. Since the second laminate of the first layer 35 of core member 20 and the laminates of the corresponding layer of core member 19 have essen-' tially the same dimensions as laminate 11, they can be.

formed by repeating the same indexing, bending and.

shearing steps.

Thus, the laminates for both or one of the core members 19, 20 can be initially preformed. The laminates. 11a and 12a of the second layer are formed with leg portions having the lengths that differ by essentially one thickness from the lengths of the corresponding or adjacent leg portions of the laminates 11, 12 in the first layer 35. Similarly, the laminates 11b, 12b of the third layer are formed by increasing each of the leg lengths by essentially one thickness of the strip material, preferably, as measured before the laminates are fabricated. The laminates 11c, 12c, which form the fourth layer of laminate layer group 25 have leg portions with lengths that differ by one strip thickness respectively from the lengths of the corresponding leg portions of the laminates inthe preceding third layer.

The finst pair of laminates 50, 5 1 of the second laminate layer group 26 is formed with leg portions 52, 56 and 54, 55 having their lengths respectively increased by essentially five strip thicknesses over the lengths of the adjacent leg pontions of laminates 11c and 12c. The successive pair of laminates in the second laminate layer group 26 are formed by increasing the leg lengths of the laminates in each successive layer by essentially one thickness over the corresponding lengths of the leg portions of the laminates in the preceding layer of the same group. The successive laminate groups 27, 28, 2-9 and 30 are formed in the same manner by varying the lengths of the successively formed laminates.

From the foregoing description it will be apparent that the particular stepped joint configuration achieved depends on the manner in which the lengths of the leg portions of the preformed laminates are altered. For example, if a single layer stepped joint is desired, the lengths of the leg portions of the successive laminates are progressively increased by an increment essentially equal to two actual thicknesses of the strip material.

In FIGURE 6 I have illustrated an assembly of a magnetic core 60 and electrical coils 61, 62 and 63, such as may be used in three phase power applications. The magnetic core 60 is comprised of two essentially rectangular core members 64 and 65 disposed in side by side relationship and enclosed or boxed in by a rectangular core member 66. It will be seen that in this core construction a two-layer stepped joint configuration is employed. The laminates 67 and 68 of the first layer of core member 65 are formed with predetermined lengths to provide the desired window space in the core 60. When the laminates 67 and 68 are joined to form the inner layer 69, an overlap of essentially one thickness is provided at the ends. The laminates 67a and 68a of the second layer are formed by increasing each of the leg portions by essentially one thickness over the lengths of the corresponding leg portions of the first layer 69. When the laminates 67a, 6811 are nested over the laminates 67, 68 the first laminate layer group is formed. The first layer of the second laminate group is formed by increasing the lengths of leg portions by three thicknesses over the lengths of the leg portions of the adjacent laminates 67a, 68a in the second layer of the first laminate group. The laminates of the next layer or second laminate layer of the second laminate layer group are formed by increasing the lengths of the leg portions of the laminates each by one thickness. The successive laminate layer groups of core member 65 are then similarly formed.

It will be seen that the adjoining core member 64 is identical to core member 65. After the two core members 64 and 65 are placed side-by-side, the laminate layer groups of core member 66 are assembled over core members 64 and 67. A resin adhesive was applied between the stepped joints to maintain the core 60 in assembled relation.

In the core constructions embodying the invention, each leg portion of the laminates in a successive layer of a given laminate group difiers essentially by one thickness from the length of the adjacent leg portion of the laminate in the preceding laminate layer. Preferably, the first and second leg portions of the laminates in the first layer of a successive laminate group are formed with a length that differs from the length of the adjacent first and second leg portions of the laminates in the last layer of the preceding laminate group by (n+1) strip thicknesses, where n is the number of layers per laminate group.

In FIGURE 7 I have illustrated curves representing a plot of flux density in kilolines per square inch versus core loss in watts per pound of the cores for three transformers each having a five k.v.a. rating. Curve A shows the core losses in a conventional transformer utilizing a core with E1 laminations. At a flux density of a kilolines per square inch, the total core loss in watts per pound was approximately 1.21.

Curve of FIGURE 7 represents a plot of flux density against the total core loss in watts per pound for a comparable 5 k.v.a. transformer having a core constructed of preformed L-shaped laminates with a single layer stepped joint configuration. At 105 kilolines per square inch, the total core loss per pound was approximately 1.28 watts per pound. Curve C represents the loss characteristic of a transformer employing the improved magnetic core construction shown in FIGURE 1 with a four-layered stepped joint pattern. In this core construction the loss in watts per hour at a flux density of 105 kilolines per square inch was found to be approximately 1.072. Thus, with the four layer stepped joint configuration, it is possible to obtain a 11.4 percent reduction in the core losses as compared with a conventional transformer magnetic core. As compared with the 5 k.v.a. transformer utilizing a single layer stepped joint, it was possible to achieve approximately 16 percent reduction of the total core loss per pound.

From the foregoing description, it will be apparent that in the improved core construction not only does the maximum flux pass in the direction of most favorable magnetic permeability but also a path of low magnetic reductance is provided at the joints between sections of the core members. It will be appreciated that magnetic strip material, such as is used in the fabrication of preformed cores, comes from the mill with certain specified thicknesses which may range from .0125 mil to .0145 mil. However, the material as it comes from the roll actually will vary several mils from the specified thickness. By the provision of a multi-layer stepped joint pattern wherein the laminates of each group layer are butt-jointed and nested with the successive layer laminate groups to provide the multi-layer stepped joint pattern, the joint can be readily assembled Without gaps. Further, irregularities in thickness are effectively taken up in a given layer group without impairing the nesting of the adjacent laminate layers of the other laminate layer groups.

Although in the specific exemplifications of my invention, I have shown magnetic cores in which a two and a four stepped joint configuration were employed, it will be appreciated that other multiple layer stepped joint arrangements can be utilized. Further, it will be understood that core members utilizing the improved arrangement can be variously combined to build up core structures of a desired configuration. It will be apparent that many modifications of the invention described herein may be made. I intend therefore by the appended claims to cover all such modifications that fall within the true spirit and scope of the invention.

What I claim as new and Patent of the United States is:

1. In a core structure for electrical inductive apparatus, a core member comprising a plurality of successively superposed laminate layer groups, each of said laminate layer groups including at least one essentially rectangular laminate layer and each of said laminate layers formed of a pair of preformed laminates, each laminate having a pair of leg portions formed of a bent strip of magnetic material having the most favorable magnetic direction lengthwise of the strip, each leg portion of the laminates in a successive layer of a laminate layer group having a length that differs by one thickness of the magnetic madesire to secure by Letters terial from the length of the adjacent leg portion of the laminate in the preceding layer, and each leg portion of the laminates of the first layer of a successive laminate layer group having a length that differs from the length of the adjacent leg portion of the laminates in the last laminate layer of the preceding laminate layer group by an amount substantially equal to the number of layers in said successive laminate group multiplied by the thickness of the magnetic material plus one additional thickness.

2. In a core structure for electrical induction apparatus, a plurality of successively superposed rectangular layer groups, each of said rectangular laminate layer groups formed of n laminate layers, where n is an integer greater than one, each of said laminate layers including a pair of laminates, each laminate having a first and a second leg portion disposed in an essentially normal relation with respect to each other and formed from magnetic strip material having the most favorable magnetic direction lengthwise of the strip, the first and second leg portions of a laminate in the first laminate layer of a successive rectangular laminate layer group having a length that differs from the length of the adjacent first and second leg portions of the laminates in the adjacent layer of the preceding rectangular laminate layer group by an amount equal to essentially (n+1) thicknesses of the magnetic strip material, and the first and second leg portions of the laminates in a successive laminate layer in any laminate layer group having lengths that differ respectively from the lengths of the adjacent first and second leg portions of the laminates in a preceding laminate layer by one thickness of the magnetic strip material.

3. A core member comprising a plurality of successively superposed laminate layer groups, each of said laminate layer groups comprised of laminate layers, each of said laminate layers including a pair of preformed laminates, each laminate having a pair of leg portions formed of a bent strip of magnetic material having the most favorable magnetic direction lengthwise of the strip, said leg portions of a laminate being disposed in essentially normal relation with respect to each other, one set of laminates of the pairs of laminates in a laminate layer group being nested to define a first L-shaped core section and the other set of the laminates of said last-mentioned pairs of laminates in a laminate group being nested to define a second L-shaped core section, said first and second L-shaped core sections being joined so that one end of the first laminate of the first L-shaped core section overlaps one end of each of the laminates of the second L-shaped core section and one end of the first laminate of the second L-shaped core section overlaps one end of each of the laminates of the first L-shaped core section to form a rectangular shaped laminate layer group, and successive laminate layer groups being similarly formed of a plurality of rectangularshaped laminate layer groups With the joints between the core sections forming a diagonally extending stepped joint pattern.

4. In a core structure for electrical induction apparatus, a plurality of successively superposed laminate layer groups, each laminate layer group formed of n laminate layers, where n represents an integer greater than one, each laminate layer including a pair of L-shaped laminates, each L-shaped laminate having a first and second leg portion and formed of magnetic strip material having its most favorable magnetic direction along the length of the strip, the first and second legs portions for the L- shaped laminates in a successive laminate layer of each laminate layer group being formed with a length that differs from the length of the corresponding first and second leg portions of the L-shaped laminates in the preceding laminate layer by one thickness of the magnetic strip material, the first laminate layer of each successive laminate layer group having a pair of laminates with a first and second leg portion that differs by (n+1) thicknesses of said magnetic strip material from the corresponding lengths of the first and second leg portions of the adjacent L-shaped laminate of the preceding laminate layer group, and the ends of said L-shaped laminate defining a stepped joint pattern.

5. In a laminated core structure for an electrical induction apparatus, a plurality of magnetic core members, each of said core members comprising a plurality of successively superposed laminate layer groups, each of said rectangular laminate layer groups having 11 layers, It representing an integer greater than one, each of said layers formed of a pair of laminates, and each of said laminates having a first and second leg portion substantially defining a ninety degree angle therebetween, the lengths of the corresponding leg portions of the laminates in any one layer being essentially equal, the lengths of the first and second leg portions of the laminates in a successive laminate layer differing in length by one thickness from the length of the adjacent first and second leg portions of the laminates in the preceding laminate layer, and the lengths of the first and second leg portions of the first laminate layer of each successive layer group differing in length from the adjacent first and second leg portions of the last laminates of the preceding laminate layer by (n+1) thicknesses of the magnetic strip material.

6. In a core structure for electrical inductive apparatus, at least one core member comprising a plurality of successively superposed laminate layer groups, each of said laminate layer groups including at least one laminate layer, each of said laminate layers including a pair of L-shaped laminates, each L-shaped laminate having a pair of leg portions, one set of L-shaped laminates of the pairs of L-shaped laminates in a laminate layer group being nested to define a first L-shaped section and the other set of the laminates of said last-mentioned .pairs of laminates being nested to define a second L- shaped core section, said first and second L-shaped being assembled so that one of the first L-shaped laminate of the first L-shaped core section overlaps one end of each of the L-shaped laminates of the second L-shaped core section and one end of the first L-shaped laminate of the second L-shaped core section overlap-s one end of each of the L-shaped laminates of the first L-shaped core section thereby to form a rectangular shaped laminate layer group, and successive laminate layer groups being similarly formed of a plurality of rectangular shaped laminate layer groups with the joints between the core sections forming a diagonally extending stepped joint pattern.

7. A core structure for electrical inductive apparatus, said core structure comprising a pair of core members disposed in side by side relation, each of said core members formed of a plurality of successively superposed laminate layer groups, each of said laminate layer groups including at least one laminate layer, each of said laminate layers comprising a pair of preformed laminates, each laminate having a pair of leg portions formed of a thin strip of magnetic material, each leg portion of the laminates in a successive layer of a laminate layer group having a length that differs by one thickness of the magnetic material from the length of the adjacent leg portion of the laminate in the preceding layer, and each leg portion of the laminates of the first layer of a successive laminate layer group having a length that differs from the length of the adjacent portion of the laminates in the last laminate layer of the preceding laminate layer group by an amount substantially equal to the number of layers in the successive laminate group multiplied by the thickness of the magnetic material plus one additional thickness.

8. A core structure for electrical induction apparatus comprising: a first and a second core member disposed in side by side relation, a third core member disposed aroundsaid first and second core members, each of said core members comprising a plurality of successively superposed rectangular laminate layer groups, each of said :rectangular laminate layer groups formed of n laminate layers, Where n represents an integer greater than one, each of said laminate layers including a pair of laminaates, each laminate having a first and a second leg portion, the first and second leg portions of the laminate in the first laminate layer of a successive rectangular laminate layer group having a length that differs from the length of the adjacent first and second leg portions :of the laminates in the adjacent layer of the equal to essentially (n+1) thicknesses of the magnetic strip material, and the first and second leg portions of the laminates in a successive laminate layer of each laminate layer group having a length that differs respectively from the lengths of the adjacent first and second leg portions of the laminate in a preceding laminate layer by one thickness of the magnetic strip material.

No references cited.

preceding rectangular laminate layer group by an amount 10 LEWIS H. MYERS, Primary Examiner. 

1. IN A CORE STRUCTURE FOR ELECTRICAL INDUCTIVE APPARATUS, A CORE MEMBER COMPRISING A PLURALITY OF SUCCESSIVELY SUPERPOSED LAMINATE LAYER GROUPS, EACH OF SAID LAMINATE LAYER GROUPS INCLUDING AT LEAST ONE ESSENTIALLY RECTANGULAR LAMINATE LAYER AND EACH OF SAID LAMINATE LAYERS FORMED OF A PAIR OF PREFORMED LAMINATES, EACH LAMINATE HAVING A PAIR OF LEG PORTIONS FORMED OF A BENT STRIP OF MAGNETIC MATERIAL HAVING THE MOST FAVORABLE MAGNETIC DIRECTION LENGTHWISE OF THE STRIP, EACH LEG PORTION OF THE LAMINATES IN A SUCCESSIVE LAYER OF A LAMINATE LAYER GROUP HAVING A LENGTH THAT DIFFERS BY ONE THICKNESS OF THE MAGNETIC MATERIAL FROM THE LENGTH OF THE ADJACENT LEG PORTION OF THE LAMINATE IN THE PRECEDING LAYER, AND EACH LEG PORTION OF 